God gave Paul great success in bringing many to Christ with the gospel in Ephesus, and had great hope of bringing many more to Christ, and for this very reason, he's now experiencing opposition. 

1Corinthians 16:9(KJV) "For a great door and effectual is opened unto me, and there are many adversaries." 

The opened door speaks of an opportunity that is a great advantage for Apostle Paul to minister to more people in Ephesus. Jesus is the Door, and He knows how to open doors and how to shut doors which are not His Will for our lives.

Revelation 3:8 "“I have set before thee an open door, and no man can shut it”

• He opened the 'Door of Faith' - Acts 14:27c ".....and how he had opened the door of faith unto the Gentiles."
• He opened the 'Door of 'Hope' - Hosea 2:15a "¹⁵ And I will give her her vineyards from thence, and the valley of Achor for a door of hope...." 
• He opened the 'Door of Utterance' - Colossians 4:3 "³ Withal praying also for us, that God would open unto us a door of utterance, to speak the mystery of Christ, for which I am also in bonds:"

The devil will oppose those with great destinies and  to be used to expand and move the Kingdom of God forward on earth.  A glorious door to fulfill destiny - A door to greatness and a life of fulfillment - A door to being an eternal excellency and bringing joy to lives of many generations coming. 

CHARACTERISTICS OF THE DOOR OPENED

1. The door is great in significance, to bring in lost souls, for that was the last commandment. - Mark 16:15 ^15"And he said unto them, Go ye into all the world, and preach the gospel to every creature."
2. The Door is to used effectively to minister the gospel, because there was work to be done, for God has given us the power to finish it. The door to do great exploits for the kingdom is swung wide open, and we as labourers in the Vineyard should go forth with the message of salvation to all who would hear it.
3. There are foes and adversaries at the door, these could be religious people, ritualist, opposition from family members and friends.

Whenever we experience opposition, it means we are to diligently press through. No adversaries and opposition can or should break the spirits of faithful and fervent Believers, but rather only rekindle their zeal, and inspire them with fresh courage.

Paul was chosen to go through the door. He was challenged on all sides. He was comitted to finish the work and receive the crown that's laid wait for him.

We have all been called and  chosen! We must arise to the  Challenge and we must commit ourselves to go through the  Opened Door!